(1) Field of the Invention
The present invention relates to blended ethylene polymer compositions and to methods for making them. More particularly, the present invention relates to ethylene polymer compositions of recycle high density polyethylene and low density polyethylene blended together with a compatabilizer and to methods for making the compositions.
(2) The Prior Art
Low density polyethylene resin is used for many purposes including the manufacture of foam products which were useful as cushioning materials and films for converting to packaging material, known as bubble wrap. Attempts have been made to incorporate a high density polyethylene, such as that obtained from recycling the high density polyethylene resins used to make milk jugs, into the low density polyethylene resins typically used to make the cushioning foams and packaging films. The efforts to blend high density polyethylene and low density polyethylene are hampered because the high density polyethylene has a significantly higher melting point temperature than the low density polyethylene.
It is known that the structural difference in polyethylenes results in notable property differences. For example, highly branched low density polyethylene has a melting point of about 108.degree. C. versus about 130.degree. C. for linear high density polyethylene. This difference causes processing difficulties and quality problems when cooling the high density polyethylene, which tends to solidify at a temperature above the low density polyethylene. This difficulty becomes more pronounced in the relatively low temperature foam manufacture. The necessary cooling brings melt temperature significantly below the high density polyethylene's melting point. The uneven solidification causes scale lines (known as fish scales) and irregular bumps on the final product.
In general, compatibility becomes critical when the level of high density polyethylene is close to 5% in foam manufacture and 15% for film manufacture. A common practice to improve compatibilization is to include linear low density polyethylene (LLDPE) into the compositions because LLDPE's long chain and short branching makes it a structurally sound buffer for the blend. However, LLDPE does not possess as good a melt strength as low density polyethylene and the associated heat generation and extrusion makes for a lower quality product. In cast film operation, for example, presence of linear low density polyethylene poses draw resonance concern as the result of insufficient branching, and the production speed is limited. In brief, linear low density polyethylene alone as a replacement for low density polyethylene when blending with a high polyethylene provides marginal overall benefit in foam as well as film operation.
As the desire for higher level of recycled resin in commodity plastics increases, lots of effort is being spent on upgrading processing and equipment to produce blended compositions containing the recycled resins. By way of example, U.S. Pat. Nos. 5,096,046 and 5,088,910 both to Goforth et al. disclose systems for producing synthetic wood products containing recycled polyethylene blended materials. The Goforth et al. patents recognize the compatibility problems created in mixing low density and high density polyethylenes together. Goforth et al. further recognize that compatibilizers may be added to polyethylene resins blends. Other prior art such as U.S. Pat. No. 5,090,662 to Banerjie discloses construction materials which include blends of recycle polymeric materials and the use of compatibilizers, such as ethylene-propylene-maleic anhydride copolymers, styrenemaleic anhydride and the like in minor amounts, to make better polyethylene blends.
It is therefore a general object of the present invention to provide a composition of high density polyethylene together with low density polyethylene suitable for making aesthetically appealing foam products for use as a cushioning materials and film products for use in the packaging industry.
It is another object of the present invention to provide a method for producing a polyethylene composition of low density polyethylene and recycled high density polyethylene by blending together with a compatibilizer.
Other objects, features and advantages of the invention will be apparent from the following detailed description of the invention as more fully described.